Handover in a mobile communication system using conference facility

ABSTRACT

A communications network comprises a central controller which operates to control a zone of the network and which provides a conference call facility with at least one gateway engaged in a call with a mobile station via a first communication channel, means for generating a hand-off required indication indicating that handover of the call is needed, means at the central controller for receiving said hand-off required indication and opening a conference call channel as a second communication channel for the call and means for handing over said call from the first to the second communication channels and, if the handover is successful, causing the first communication channel to be closed.

The present invention is related to handovers in a mobile communicationsystem and particularly, but not exclusively, to mobile stationhandovers in an IP-based telecommunications network.

Prior art office-based communications systems usually operateconventional fixed-line telephone units linked via an internalswitchboard or PBX (private branch exchange). Such fixed-line systemsare able to provide relatively high voice quality. However, usermobility is severely impaired.

The advent of digital mobile technologies such as GSM, however, meansthat mobile systems can now provide equivalent, if not higher, voicequality than fixed-line systems. Mobile systems also allow greaterfreedom of movement for the user within the office than do fixed-linesystems.

WIO (Wireless Intranet Office) is a proprietary communications systemdeveloped by the applicants which introduces the concept of utilisingmobile telephone units, such as conventional GSM mobile stations, in anoffice environment. The system makes use of a known concept calledInternet Telephony or Voice-over-IP.

Voice-over-IP is a technology which allows sound, data and videoinformation to be transmitted over existing IP-based Local or Wide AreaNetworks or the Internet. The technology thus provides for convergenceand integration of three different media types over the same network.

Prior to the advent of Voice-over-IP, offices often operated threeseparate networks for the transmission of these media types. Asindicated above, fixed-line telephone systems coupled to an in-house PBXprovided for voice communication, an office-based LAN or Intranet (i.e.a packet-switched internal network), comprising computer terminalslinked via network cards and under the control of a server station,provided for the transmission of “conventional” computer data and videocameras linked to monitors via fixed line or remote transmission linkprovided for video communication.

Voice-over-IP effectively combines these three media types such thatthey can be transmitted simultaneously on the same packet-switchednetwork or IP-router throughout the office environment and beyond theconfines of the office.

In order to provide for such media convergence, Voice-over-IP often usesa specific ITU (International Telecommunication Union) a standardprotocol to control the media flow over the Intranet. One commonstandard protocol used in Voice-over-IP systems, and the one used in theWIO system, is termed H.323.

H.323 is an ITU standard for multimedia communications (voice, video anddata) and allows multimedia streaming over conventional packet-switchednetworks. The protocol provides for call control, multimedia managementand bandwidth management for both point-to-point (2 end-users) andmultipoint (3 or more end-users) conferences. H.323 also supportsstandard video and audio codecs (compression/decompression methods suchas MPEG) and supports data sharing via the T.120 standard.

Furthermore, H.323 is network, platform and application independentallowing any H.323 compliant terminal to operate in conjunction with anyother terminal.

The H.323 standard defines the use of three further command and controlprotocols:

-   a) H.245 for call control;-   b) Q.931 based protocol for call signalling; and-   c) The RAS (Registrations, Admissions and Status) signalling    function.

The H.245 control protocol is responsible for control messages governingthe operation of the H.323 terminal including capability exchanges,commands and indications. Q.931 is used to set up a connection betweentwo terminals. RAS governs registration, admission and bandwidthfunctions between endpoints and gatekeepers (defined later).

For a H.323 based communication system, the standard defines four majorcomponents:

-   1. Terminal-   2. Gateway-   3. Gatekeeper-   4. Multipoint Control Unit (MCU).

Terminals are the user end-points on the network, e.g a telephone or faxunit or a computer terminal. All H.323 compliant terminals must supportvoice communications, but video and data support is optional.

Gateways connect H.323 networks to other networks or protocols.

For an entirely internal communications network, i.e. with no externalcall facility, gateways as such are not required. In that case, internalintranet mobile clusters (IMCs) act as internal gateways.

Gatekeepers are the control centre of the Voice-over-IP network. It isunder the control of a gatekeeper that most transactions (communicationbetween two terminals) are established. Primary functions of thegatekeeper are address translation, bandwidth management and callcontrol to limit the number of simultaneous H.323 connections and thetotal bandwidth used by those connections. An H.323 “zone” is defined asthe collection of all terminals, gateways and multipoint-control units(MCU defined below) which are managed by a single gatekeeper.

Multipoint Control Units (MCU) support communications between three ormore terminals. The MCU comprises a multipoint controller (MC) whichperforms H.245 negotiations between all terminals to determine commonaudio and video processing capabilities, and a multipoint processor (MP)which routes audio, video and data streams between terminals.

The conventional Voice-over-IP system described herein above normallyutilise standard fixed-line telephone systems which are subject to thedisadvantages outlined above, namely the lack of mobility and the lackof user commands.

The WIO concept takes Voice-over-IP further in that it provides for theuse of conventional mobile telephone units, such as GSM mobile stations,within the Voice-over-IP system. To provide for such mobilecommunications within an intra-office communication network, WIOcombines known Voice-over-IP, as described above, with conventionalGSM-based mobile systems.

Thus, intra-office calls are routed through the office intranet andextra-office calls are routed conventionally through the GSM network.Such a system provides most or all of the features supported by themobile station and the network such as telephone directories, shortmessaging, multiparty services, data calls, call barring, callforwarding etc. WIO, therefore, provides for integrated voice, video anddata communications by interfacing an H.323-based voice-over-IP networkwith a GSM mobile network.

The WIO system is a cellular network, similar to the conventional GSMnetwork and is divided into H.323 Zones as described above. One H.323Zone may comprise a number of cells. Two or more H.323 zones may becontained within an administrative domain. The allocation of H.323 zonesto an administrative domain is an issue primarily concerning billing andis therefore not relevant to this invention.

Given the cellular nature of the WIO system, a major issue to be solvedis that of handovers. As a mobile station moves from cell to cell itreports its location to the BTS or equivalent controller. When it movesfrom one zone to another, a handover is needed to a different gateway. Asimilar consideration applies to mobile stations in the conventional GSMnetwork.

In such conventional GSM systems, the need for a handover of a mobilestation to a different cell of the network is normally determined by themobile station measuring the strength of signals transmitted fromseveral base transceiver stations.

During the time that it is in a particular cell, the mobile stationcontinuously receives signals from several base transceiver stations inadjacent cells. It sends the measurement reports to the BTS/BSC whichmakes a decision when handover is needed. If the level of a signaltransmitted by a base transceiver station, located in a different cellfrom that of the mobile, reaches a certain threshold level T1 inrelation to that of the base transceiver station located in the mobilestation's current cell, the BSC will determine that a handover to thatcell is required and will issue a handover required indication to thenetwork controller (main switching centre).

In a similar manner, the IMC of a WIO system is able to determine therequirement for handover within the WIO network by comparing the signalstrengths of the signals reported by the mobile station.

However, added complexities arise for handovers in the WIO system sincea mobile unit operating therein is not only able to move between cellswithin the WIO system, but also between zones and even between the WIOsystem itself and an external GSM network.

It can be seen, therefore, that there are several different types ofhandovers which may need to be executed in the normal operation of a WIOsystem. These types of handovers are:

-   a) The handover of a mobile from one WIO cell to another whilst    engaged in a call.-   b) The handover of a mobile from one WIO zone to another whilst    engaged in a call.-   c) The handover of a mobile from a cell within the WIO system to a    cell within an external GSM system.-   d) The handover from a cell within an external GSM system to a cell    within the WIO system.

The call can involve a mobile station and any other endpoint orterminal, fixed or mobile.

According to one aspect of the present invention, therefore, there isprovided a communications network comprising: a central controller whichoperates to control a zone of the network and which provides aconference call facility; at least one gateway engaged in a call with amobile station via a first communication channel; means for generating ahand-off required indication indicating that handover of the call isneeded; means at the central controller for receiving said hand-offrequired indication and opening a conference call channel as a secondcommunication channel for the call; and means for handing over said callfrom the first to the second communication channels and, if the handoveris successful, causing the first communication channel to be closed.

According to another aspect of the present invention there is provided amethod of effecting handover of a call in which at least one mobilestation is engaged via a first communication channel in a communicationsnetwork comprising a central controller which operates a zone of thenetwork and provides a conference call facility, in which method: ahand-off required indication is issued indicating that handover of thecall is needed; the central controller receives said hand-off requiredindication and opens as a second communication channel a conference callchannel; handover of said call is effected from said first to saidsecond communication channel; and if the handover is successful thefirst communication channel is closed.

Thus, the present invention provides handover control using existingconference call functionality rather than having to provide newfunctionality with which to control the handover.

For a better understanding of the present invention, and to show how thesame may be carried into effect, the present invention will now bedescribed in more detail with reference to the accompanying drawings inwhich:

The means for generating a hand-off required indication is preferablycomprised in the gateway or, for example, a mobile station.

The means for handing over may be a gatekeeper.

The communications network may be a cellular network.

FIG. 1 a is a block diagram showing some of the components used in theimplementation of a WIO system;

FIG. 1 b is a block diagram illustrating the communication pathways usedduring a call between an internal mobile station and an external mobilestation;

FIG. 1 c is a block diagram illustrating the communication pathways usedduring a call between two internal mobile stations operating under thesame gatekeeper;

FIG. 1 d is a block diagram illustrating the communication pathways usedduring a call between two internal mobile stations operating underdifferent gatekeepers;

FIG. 2 is a block diagram illustrating the communication pathwaysbetween components of the WIO system before the mobile station handoveraccording to a first embodiment;

FIG. 3 is a block diagram illustrating the communication pathwaysbetween components of the WIO system during a first period of the mobilestation handover according to a first embodiment;

FIG. 4 is a block diagram illustrating, the communication pathwaysbetween components of the WIO system during a second period of themobile station handover according to a first embodiment;

FIG. 4 a is a diagram illustrating the multiplexing within the MCU; and

FIG. 5 is a block diagram illustrating the communication pathwaysbetween components of the WIO system during a third period of the mobilestation handover according to a first embodiment.

Reference is first made to FIG. 1 a to describe the context of thisinvention.

A WIO system can be provided in an office and operator environment basedon an IP (Internet Protocol) based packet network. In practice there aretwo Local Area Networks (LANs) or Intranets which are operable to carrystandard packet-switched data. These are connected via, e.g., a widearea network (WAN). An IP (Internet Protocol) router 10 can be used inthe LAN environment as can an IP switch or hub. The office environmentis denoted by a dashed box 100.

One or more mobile stations (MS) 1 communicate, i.e. transmit signals toand/or receive signals from, a base transceiver station (BTS) 2. Thebase transceiver station 2 used in the WIO system is similar to basetransceiver stations used in conventional GSM mobile communicationssystems in that it is connected to, and operates in conjunction with, acontroller. In a conventional GSM system, the controller is termed abase station controller (BSC); in WIO, however, the controller isrepresented by an Intranet Mobile Cluster interface (IMC) 3, thefunction of which will be described later. Also, some of the functionsof a conventional BSC are located in an A-interface gateway 8 or WIOgatekeeper 4.

The base transceiver station 2 therefore receives signals transmitted bythe mobile unit 1 and forwards them to the Intranet Mobile Clusterinterface 3. The Intranet Mobile Cluster interface 3 is also connectedto the IP-router 10.

A WIO Gatekeeper (WGK) 4 is connected to the IP-router 10 as is an H.323terminal (H.323) 5. The H.323 terminal 5 may be represented by acomputer terminal which supports voice information or an H.323application.

One or more telephone or facsimile units (T/F) 12 are connected to aprivate branch exchange (PBX) 6 as in a conventional fixed-line officecommunication system. The private branch exchange 6 is connected to theIP-router 10 via an interface (INT) 7. The private branch exchange 6 isadditionally connected to a Public Switched Telephone Network (PSTN) 13such as a conventional BT network.

The IP-router 10 is connected to an external IP-based network 30, suchas a Wide Area Network (WAN) or the Internet.

For extra-office communication, the IP-based network 30 is connected toa second IP-router 20, on the operator side, which may also berepresented by a LAN or Intranet.

Connected to the second IP-router are two interfaces, an A-interfacegateway (AGW) 8 and an ISDN gateway (IGW) 11, and an intranet locationregister (ILR) 9. The AGW 8, ISDN gateway 11 and Intranet locationregister 9 are also connected to a standard GSM network as used in aconventional mobile communications system. The ISDN gateway isadditionally connected to the Public Switched Telephone Network (PSTN)13.

WIO allows for the use of mobile telephone in the office environment tomake both intra- and extra-office calls. The functions of each of thecomponents of FIG. 1 will now be described in more detail.

The Intranet Mobile Cluster interface 3 performs similar functions tothat of a base station controller (Base Station Controller) in aconventional GSM network such as the management of radio resources andchannel configuration and the handling of the Base transceiver stationconfiguration. However, the intranet Mobile Cluster interface alsoprovides conversion from GSM voice data to packet based data suitablefor transmitting on the packet-based IP Router 10.

During a call, therefore, the Intranet Mobile Cluster interface 3converts the voice data transmitted by the base station intopacket-based data suitable for transmitting on the IP Router 10.

The WIO Gatekeeper 4 is the main controller of the WIO system and has ahigh processing capability. Its function is to provide control servicesto the IP Router and the various user terminals. However, the Gatekeeper4 is also responsible for all of the functions which the H.323 protocoldefines to its gatekeeper, including call management and callsignalling. The WIO Gatekeeper 4 is able to manage the main differentcall types such as voice, data, facsimile and conference calls which canbe established between a mobile station, a PC terminal and a normaltelephone in any combination.

The A-interface gateway 8 handles communication between the WIOenvironment and the GSM network via the mobile services switching centre(MSC). From the MSC viewpoint, the WIO appears to be a conventional basestation controller. The A-interface gateway 8 also provides an interfacefor an Operations and Maintenance Unit (O&M—not shown).

The ISDN gateway (IGW) 11 handles communication between the WIOenvironment and the public telephone network and has an interface toboth the MSC and the PSTN 13. The ISDN gateway 11 is used to enablecommunication mainly from a non-GSM H.323 terminal to a GSM mobile unitoutside the WIO network. It also provides for the possibility ofcommunication from a mobile station to a PC. Only GSM mobile stationscan be connected to the MSC via the A-interface gateway 8. The WIOGatekeeper 4 controls the status of the ISDN gateway 11 and sends theinformation to the O&M unit.

The intranet location register 9 provides the directory services in theWIO system. The purpose of the directory service is to provide storagefor mobile station-specific information, such as mobile stationidentities and supplementary services such as billing accounts for usein the WIO system. All of the mobile stations within the WIO system havean entry in the intranet location register. The WIO Gatekeeper 4 updatesthe current intranet location register entry of each mobile stationallowing the correct mobile number to be accessed in each call. Theintranet location register is linked to the home location register (notshown) within the GSM network via a mobile application part (MAP)interface (not shown).

The telephone calls managed by the WIO system can be divided intointernal calls and external calls. Internal calls involving a mobilestation are those calls where the mobile station and the other terminalinvolved in the call are located within the WIO system, and externalcalls involve a mobile station or other terminal which is not locatedwithin the WIO system. Calls may also be between H.323 terminals andexternal PSTN subscribers.

The functions of the WIO system components described above will now bedescribed, with reference to FIG. 1 b, in the context of a telephonecall from a mobile station located within the WIO system (mobile A) to amobile station located in an external network such as a GSM network(mobile B).

The mobile station A transmits a radio frequency (RF) transmissionsignal TX, on a predetermined communication channel, to the basetransceiver station 2 in a format conventional to GSM communicationssystems such as a time-slot format. The communication charnel on whichthe mobile station A transmits the RF transmission signal TX isdetermined in a manner conventional to GSM communication systems.

The base transceiver station 2 receives the RF transmission signal,down-converts it and then forwards it to the intranet mobile clusterinterface 3. In this respect, the base transceiver station 2 and theintranet mobile cluster interface 3 operate in a manner similar to abase transceiver station and a base station controller respectively in aconventional GSM network.

The intranet mobile cluster interface 3 receives the down-convertedtransmission signal from the base transceiver station 2 and converts thevoice data from the conventional GSM time-slot format, to a packet-basedformat which allows it to be transmitted along the IP-router 10. This isreferred to herein as the PAYLOAD. Also, the intranet mobile clusterinterface 3 composes a control signal CTRL containing call signallingmessages, for example identification of the destination MS B, the IPaddress corresponding to that MS and/or identification of the source MSA.

The control signal CTRL is then routed, in packet format, via theIP-router 10, to the gatekeeper 4 which, based on the informationcontained in the control signal CTRL, determines whether the mobilestation B is located within the WIO system or external to the WIOsystem. If the mobile station B lies outside the WIO system, e.g. aconventional GSM mobile unit operating in the GSM network, thegatekeeper 4 causes the control signal CTRL and the payload signalPAYLOAD to be routed along the IP-router 10 to the second IP-router 20via the external network 30.

The payload signal PAYLOAD, still in packet format, is transmitted fromthe second IP-router 20 to the A-interface gateway 8. The A-interfacegateway converts the payload signal from the packet-based format used totransmit it on the IP-routers and Internet (10, 20, 30) into aconventional GSM format such as a time-slot format.

The receive signal RX, in timeslot format, is forwarded from theA-interface gateway 8 to the Mobile Services switching centre 26 (MSC)in the GSM Network from where it is transmitted to the respective BaseStation Controller/Base transceiver station 24, under which the mobilestation B is operating, in a manner conventional to GSM.

Calls which are completely internal to the WIO system are handledslightly differently as shown in FIG. 1 c. The RF transmission signalTX, in timeslot format, transmitted by the mobile A is again sent to theBTS 2 which performs down conversion of the signal. The down-convertedsignal is forwarded to the intranet mobile cluster interface 3 whichperforms format conversion to generate a PAYLOAD packet and a CTRLpacket.

From the intranet mobile cluster interface 3, the control signal CTRL issent to the Gatekeeper 4 which determines if the mobile station B iswithin the WIO system and, if so, in which H.323 Zone it is located. AH.323 zone is defined as the collection of all terminals, gateways andmultipoint control units managed by a single gatekeeper.

If the mobile station B is operating in the same H.323 Zone as themobile station A, i.e. under the same Gatekeeper which then starts thepaging procedure, the Gatekeeper 4 will receive a paging response fromthe destination intranet mobile cluster interface 23, i.e. the intranetmobile cluster interface under which the mobile station B is operating,and then routes the payload along the IP-router 10 to that destinationintranet mobile cluster interface.

The destination intranet mobile cluster interface 23 converts thepayload signal into a timeslot format. It is then sent, via its basetransceiver station 24 which performs up-conversion to RF, to the mobilestation B.

As shown in FIG. 1 d, 1 f the gatekeeper 4 determines that the mobilestation B is in a different H.323 Zone to the mobile station A, thegatekeeper 4 routes the CTRL signal to the destination gatekeeper 4′,i.e. the gatekeeper under which the mobile station B is operating. Ifthe destination gatekeeper receives a paging response from one of theintranet mobile cluster interfaces, it then routes the call signal tothe intranet mobile cluster interface and out to the mobile station B,via its base transceiver station 24, in a manner similar to thatdescribed above.

FIGS. 2 to 5 show, respectively, the communication pathways before,during and after a handover according to an embodiment of the presentinvention. The following description illustrates the handover of amobile station A from a first cell of the WIO network to a second cellof the WIO network whilst in communication with a mobile station B alsolocated within the network.

In this context, the components of the first cell, i.e. the cell out ofwhich mobile station A will move, are termed the source components whilethe components of the second cell, i.e. the cell into which mobilestation A will move, are termed the target components. The components ofthe cell in which mobile station B is located are termed the destinationcomponents. Note that the destination mobile station could be any otherendpoint or terminal such as fixed phone, fax etc.

As shown in FIG. 2, before the handover, mobile station A communicateswith mobile station B by transmitting an RF, timeslot-based signal tothe source base transceiver station 2 which down-converts the signal andsends it to the source intranet mobile cluster interface 3. The sourceintranet mobile cluster interface converts the voice data into apacket-based format (PAYLOAD) and sends it, via the MCU 28, to thedestination IMC 23. A CTRL message is formulated and sent to thegatekeeper 4. The gatekeeper 4 identifies from the CTRL message thedestination terminal as the mobile station B and then sends a furtherpacket-based signal CTRL, via the IP-router 10, to the destinationintranet mobile cluster interface 23. The destination intranet mobilecluster interface 23 converts the PAYLOAD into GSM timeslot format andforwards it to the destination base transceiver station 24 whichup-converts the signal to RF and transmits it to mobile station B.

When the IMC 3 determines, in the manner described earlier, that ahandover of MSA to another cell is required the IMC issues handoverrequired indication HRI. The IMC 3 also generates, based on the levelsof the signals the MSA has reported as having been received from thesurrounding base transceiver stations, a list of suitable target basetransceiver stations 22 to which the call could be handed over.

The gatekeeper 4 is provided by the HRI with a candidate list which is alist of cell identifiers (i.e. local Area Code and cell identifier)which the gatekeeper resolves into a choice of target gateways. For thepurposes of the following description it is assumed that all the callsare switched via the MCU 28. However Q931 signalling (CTRL) does not gothrough the MCU. The gatekeeper 4 controls the MCU via MC-CTRL packets.

When the gatekeeper 4 receives the HRI message and the candidate listfrom the source intranet mobile cluster interface 3, it selects a targetIMC 21 from the candidate list.

The gatekeeper 4 then invites the target intranet mobile clusterinterface 21 to connect to the multipoint control unit 28 so as to forma multipoint conference. In this manner, a communications link betweenthe target intranet mobile cluster interface 21 and the multipointcontrol unit 28 is set up by the gatekeeper 4. However, the existinglink between the source mobile cluster interface and the multipointcontrol unit 28 is not disconnected at this time.

From FIG. 4 it can be seen that once the communications link between thetarget intranet mobile cluster interface 21 has been set up by thegatekeeper 4, the mobile station A is, in fact, communicating with themultipoint control unit 28 via two separate communications links:firstly via the source base transceiver station and the source intranetmobile cluster interface and secondly via the target base transceiverstation 22 and the target intranet mobile cluster interface 21.

As illustrated in FIG. 4 a, the multipoint control unit 28 is operableto multiplex the PAYLOAD packets input to it. Signals transmitted by themobile station A are input to it twice firstly (S) from the sourceintranet mobile cluster interface 3 and secondly (T) from the targetintranet mobile cluster interface 21. These signals are multiplexedtogether and sent to the destination mobile station B via IMC 23.Similarly, the signal (D) transmitted by the destination mobile stationB is input to the multipoint control unit 28 from the destinationintranet mobile cluster interface 23 and is routed by the multipointcontrol unit 28 both to the source intranet mobile cluster interface 3and to the target intranet mobile cluster interface 21.

There is in effect, therefore, a three-way multipoint conference takingplace with the three endpoints being the mobile station A in the sourcecell, the mobile station A in the target cell and the mobile station Bin the destination cell. Such a multipoint conference is known per seand is described in H.323 protocol.

Once the three-way multipoint conference has been established and thecall is successfully connected at the target IMC, the gatekeeper 4 thenoperates to disconnect the original communications links between thesource intranet mobile cluster interface 3 and the multipoint controlunit 28 (FIG. 5).

In other words, the original communications link between the mobilestation A and the multipoint control unit 28 are not disconnected untilthe new connections have been established, i.e. handover has beeneffected to a level which is satisfactory to the gatekeeper 4. Thismeans that the original call path can continue to be utilised ifhandover is not successful at the target IMC.

At this point, the handover is considered to be completed.

In this manner, the mobile station A is handed over from a first cell ofthe WIO system to a second cell. Since the old communications links fromthe source cell are not disconnected until some time after the new linksare set up, data loss is reduced.

Thus, breaks in the packet stream are minimised reducing packet loss andimproving communications links. Also, the handover of MSA is invisiblefor user B.

1. A communications network comprising: a central controller whichoperates to control a zone of the network and which provides aconference call facility; at least one gateway engaged in a call with amobile station via a first communication channel; means for generating ahand-off required indication indicating that handover of the call isneeded; means at the central controller for receiving said hand-offrequired indication and opening a conference call charnel as a secondcommunication channel for the call; and means for handing over said callfrom the first to the second communication channels and, if the handoveris successful, causing the first communication channel to be closed. 2.A network according to claim 1, which comprises a switched packetcommunication path by means of which the means for handing over, centralcontroller and the gateway are connected.
 3. A network according toclaim 1, wherein the gateway comprises packet generating means forgenerating a packet addressed to said means for handing over includingcontrol information comprising a candidate list of alternative cells towhich the call could possible be transferred.
 4. A network according toclaim 1, wherein the gateway is operable to receive RF information froma mobile station, said RF information including call data, and toconvert said RF information into a packet for transmission to thegatekeeper.
 5. A network according to claim 1, wherein the centralcontroller is operable to multiplex the first and second communicationchannels, such that the call is conveyed simultaneously by both of saidchannels until the first communication channel is closed.
 6. A networkaccording to claim 3, wherein the gatekeeper comprises means forselecting a target gateway based on the cells in the candidate list, andfor causing said central controller to open the conference call channelto said selected target gateway as a second communication channel.
 7. Anetwork according to claim 1, which is an internal cellularcommunications network and which comprises an interface for connectionto an external network which includes an external controller such thatsaid call can comprise a first mobile station in the internal cellularcommunications network and a second mobile station in the externalnetwork.
 8. A method of effecting handover of a call which at least onemobile station is engaged via a first communication channel in acommunications network comprising a central controller which operates azone of the network and provides a conference call facility, in whichmethod: a hand-off required indication is issued indicating thathandover of the call is needed; the central controller receives saidhand-off required indication and opens as a second communication channela conference call channel; handover of said call is effectuated fromsaid first to said second communication channel; and if the handover issuccessful the first communication channel is closed.
 9. A methodaccording to claim 8, wherein the hand-off required indication comprisesa packet which is conveyed to the central controller via a switchedpacket communications path of the network.
 10. A method according toclaim 8, wherein the mobile station transmits call data as an RF signal,said call data being converted into a packet for transmission via aswitched packet communications path of the network.
 11. A methodaccording to claim 8, which comprises the step of multiplexing the callvia the first and second communication channels at the centralcontroller, prior to closing the first communication channel.
 12. Acommunications network comprising: a central controller which operatesto control a zone of the network and which provides a conference callfacility; at least one gateway engaged in a call with a mobile stationvia a first communication channel; means for generating a hand-offrequired indication indicating that handover of the call is needed;means at the central controller for receiving said hand-off requiredindication and opening a conference call channel as a secondcommunication channel for the call; and means for handing over said callfrom the first to the second communication channels and, if the handoveris successful, causing the first communication channel to be closed,wherein the central controller is operable to multiplex the first andsecond communication channels, such that the call is conveyedsimultaneously by both of said channels until the first communicationchannel is closed.